The present invention relates generally to joining graphite to graphite by brazing and, more particularly, to the brazing of graphite to graphite by employing a low-melting mixture of uranium carbide and hafnium carbide as the brazing material for producing a joint of essentially hafnium carbide.
Graphite has been found to provide a desirable structural material for use in high-temperature applications in both nuclear and nonnuclear technology. The thermal conductivity, thermal expansion and the high strength of graphite at elvated temperatures make this material particularly useful as a structural material for use in atomic vapor laser isotope separation processes. In the fabrication of the apparatus used for the laser isotope separation, relatively large structures of graphite with a high coefficient of thermal expansion are required. However, commercially available graphite articles are of insufficient size or of the wrong configurations required for the fabrication of certain components from single graphite structures. Consequently, the fabrication of graphite structures utilized in the apparatus for laser isotope separation processes necessitates the joining of graphite members or components so that a sufficiently large structure of graphite of the desired configuration can be provided for use in the fabrication of the laser isotope separation apparatus. Thermal compatibility of the graphite and the brazed joint together with resistance to the chemical attack at the joint area by uranium vapors or molten uranium are major concerns required in the fabrication of graphite by brazing.
Graphite has been previously joined together by using brazing processes. For example, in assignee's U.S. Pat. No. 3,946,932 which issued to George R. Peterson on Mar. 30, 1976, molybdenum powder is utilized as the brazing material. In this process, the molybdenum powder is placed between the fay surfaces of the graphite and converted to a carbide form with the addition of heat. The molybdenum diffused from the joint area during an annealing step to provide a joint consisting essentially of graphite. Another known process for joining graphite to graphite by brazing is the use of refractory metals as described in U.S. Pat. No. 2,979,813 issued to Morris A. Steinberg on Apr. 18, 1961. In the Steinberg patent, refractory metals such as elemental titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, and tungsten are used as brazing materials. The brazing of the graphite structures was provided by placing a refractory metal in either the form of a solid layer or as a layer of powder intermediate the fay surfaces of the graphite pieces to be joined. The joint area was then heated to convert the refractory metal to carbide of the metal and then decompose carbide for permitting the volatilization of the carbide from the joint area to provide a joint consisting essentially of graphite.
While the brazing processes described in the aforementioned patents, especially the Steinberg patent, by using elemental metals as the brazing material provide satisfactory joints between graphite pieces some process conditions render the brazing operation somewhat impractical. For example, hafnium carbide has been found to be particularly suitable for use in the laser isotope separation program due to its wettability by molten uranium. However, the fabrication of brazed joints by using hafnium requires that the heating of the joint area be near the eutectic temperature of hafnium carbide which is about 3150.degree. C. This fabrication of graphite parts requires the use of special furnaces and handling procedures to reach these high temperatures. A lower brazing temperature for joining graphite to graphite with hafnium carbide would be highly advantageous. Furnaces capable of 3000.degree. C. are typically quite small or highly specialized such that they are unsuited as a general working furnace capable of handling large shapes for brazing.